Dynamic Diselenide Bonds: Exchange Reaction Induced by Visible Light without Catalysis

Dynamic covalent bonds are extensively employed in dynamic combinatorial chemistry. The metathesis reaction of disulfide bonds is widely used, but requires catalysis or irradiation with ultraviolet (UV) light. It was found that diselenide bonds are dynamic covalent bonds and undergo dynamic exchange reactions under mild conditions for diselenide metathesis. This reaction is induced by irradiation with visible light and stops in the dark. The exchange is assumed to proceed through a radical mechanism, and experiments with 2,2,6,6‐tetramethylpiperidin‐1‐yloxyl (TEMPO) support this assumption. Furthermore, the reaction can be conducted in different solvents, including protic solvents. Diselenide metathesis can also be used to synthesize diselenide‐containing asymmetric block copolymers. This work thus entails the use of diselenide bonds as dynamic covalent bonds, the development of a dynamic exchange reaction under mild conditions, and an extension of selenium‐related dynamic chemistry.     

甲亚胺1,2—脱氢过程中化学键变化的量子拓扑研究

在IRC解析的基础上,用电子密度的量子拓扑分析方法研究了甲亚胺1,2-脱氢过程中化学键的变化.反应途径中各点化学键的拓扑性质可以清楚地反映出化学键断裂及生成过程,计算结果进一步证明该反应为协同非同步反应.本工作为研究化学反应过程提供了一种新的方法。     

Intermolecular amidation of unactivated sp2 and sp2 C-H bonds via palladium-catalyzed cascade C-H activation/nitrene insertion

This communication describes the Pd(OAc)2-catalyzed intermolecular amidation reactions of unactivated sp2 and sp3 C-H bonds using primary amides and potassium persulfate. The substrates containing a pendent oxime or pyridine group were amidated with excellent chemo- and regioselectivities. It is noteworthy that reactive C-X bonds were well-tolerated and a variety of primary amides can be effective nucleophiles for the Pd-catalyzed C-H amidation reactions. For the reaction of unactivated sp3 C-H bonds, beta-amidation of 1 degrees sp3 C-H bonds versus 2 degrees C-H bonds is preferred. The catalytic reaction is initiated by chelation-assisted cyclopalladation involving C-H bond activation. Preliminary mechanistic study suggested that the persulfate oxidation of primary amides should generate reactive nitrene species, which then reacted with the cyclopalladated complex.     

用FTIR研究环氧化天然橡胶的热老化

与天然橡胶（ＮＲ）相比,环氧化天然橡胶（ＥＮＲ）具有良好的耐油性和气密性［１,２］,并由于环氧基团而有一定的极性,同其它高聚物的相容性较好．通过环氧基团的二次反应,ＥＮＲ分子链可接上其它的功能团,满足一些特殊的用途［３,４］．对于ＥＮＲ的力学性能、共...     

Ruthenium-catalyzed conversion of sp3 C-O bonds in ethers to C-C bonds using triarylboroxines

Catalytic conversion of unreactive sp(3) C-O bonds in alkyl ethers to C-C bonds is described. Alkyl ethers bearing 2- or 4-pyridyl groups were coupled with triarylboroxines in the presence of a ruthenium catalyst. Triarylboroxines bearing a variety of functional groups including electron-withdrawing and -donating groups can be used for the reaction. No additional base was required for the coupling with the organoboron reagents, and base-sensitive groups can be tolerated. The reaction is considered to proceed via dehydroalkoxylation followed by addition of triarylboroxines to form C-C bonds.     

Transient bonds and chemical reactivity of molecules

Electron delocalization between the reagent and reactant molecules is the principal driving force of chemical reactions. It brings about the formation of new bonds and the cleavage of old bonds. By taking the aromatic substitution reaction as an example, we have shown the orbitals participating in electron delocalization. The interacting orbitals obtained are localized around the reaction sites, showing the chemical bonds that should be generated and broken transiently along the reaction path. By projecting a reference orbital function that has been chosen to specify the bond being formed on to the MOs of the reactant molecules, the reactive orbitals that are very similar to the interacting orbital have been obtained. The local potential of the reaction site for electron donation estimated for substituted benzene molecules by using these projected orbitals shows a fair correlation with the experimental scale of the electron-donating and -withdrawing strength of substituent groups. The reactivity is shown to be governed by local electronegativity and local chemical hardness and also by the localizability of interaction on the reaction site. © 1996 John Wiley & Sons, Inc.     

Iridium-catalyzed borylation of secondary C-h bonds in cyclic ethers

The borylation of secondary C-H bonds, specifically secondary C-H bonds of cyclic ethers, with a catalyst generated from tetramethylphenanthroline and an iridium precursor is reported. This borylation occurs with unique selectivity for the C-H bonds located β to the oxygen atoms over the weaker C-H bonds located α to oxygen atoms. Mechanistic studies imply that the C-H bond cleavage occurs directly at the β position rather than at the α position followed by isomerization of a reaction intermediate.     

Subtilisin-catalyzed religation of proteolyzed hen egg-white lysozyme: investigation of the role of disulfides

Background: The use of proteases to form, instead of break, peptide bonds has expanded the repertoire of techniques available for protein semisynthesis. Several groups have previously reported the use of proteases in aqueous-organic solvents to form single amide bonds within proteins, but low yields and lengthy reaction times make this an impractical approach to protein synthesis. We recently found that proteolyzed triose phosphate isomerase can be re-ligated rapidly and efficiently by subtilisin, in mixed aqueous-organic solvent systems.Results: We now report the use of subtilisin to resynthesize hen egg-white lysozyme from a mixture of its proteolyzed fragments in high yield and with rapid reaction times. This enzymatic religation can also be achieved after reduction of the four disulfide bonds present in lysozyme, with the same efficiency as that observed for the disulfide-containing proteolysis mixture.Conclusions: For egg-white lysozyme, the subtilisin religation reaction can be used to re-synthesize a proteolyzed protein even after reduction of disulfide bonds. The utility of this reaction in more generalized protein semisynthesis reactions is currently being explored.     

Synthesis of polycyclic heterocycles via a one-pot ortho alkylation/direct heteroarylation sequence

[reaction: see text] Polycyclic thiophenes and furans were synthesized using a one-pot ortho alkylation/direct heteroarylation reaction sequence. Under the optimized reaction conditions, aryl iodides were coupled with 3-(bromoalkyl)thiophenes or -furans, affording six- and seven-membered annulated ring products via formation of two C-C bonds from two aryl C-H bonds.     

Tunable delocalization of unpaired electrons of nitroxide radicals for sickle-cell disease drug improvements

Hydroxyurea is a drug recently approved to treat sickle cell diseases. Hydroxyurea benefits the patients by increasing the level of fetal hemoglobin via a nitroxide radical pathway. Here, we report an unpaired-electron-delocalization approach to tune the stability of nitroxide radicals. In this approach, the substitution by an unsaturated alkyl group containing conjugated C=C double bonds for the hydrogen on the nitrogen atom attached to the hydroxyl of hydroxyurea can significantly increase its ability to generate nitroxide radical. Furthermore, the increase can be remarkably enhanced by increasing the number of conjugated C=C double bonds. For a hydroxyurea derivative that contains two conjugated C=C double bonds, the reaction rate to generate its radical is 118 times faster than that of hydroxyurea, and for a hydroxyurea derivative containing 20 conjugated C=C double bonds, the reaction rate to form its radical is 238 times faster than that of hydroxyurea. For this reason, hydroxyurea derivatives with conjugated C=C double bonds may constitute new potential drugs for the treatment of sickle-cell diseases.     

Unusual kinematics-driven chemistry: cleaving C-H but not COO-H bonds with hyperthermal protons to synthesize tailor-made molecular films

Unconventional reaction-design strategies have been developed to exploit the intriguing kinematics that occur when adsorbed organic molecules are bombarded by a beam of hyperthermal protons: kinematic energy transfer is only effective in H-->H collisions and thus only C-H bonds are cleaved. This process yields a cross-linked molecular film with its chemistry governed by the selection of appropriate precursor molecules. Unlike the conventional wet-chemistry synthesis of cross-linked polymeric films, this new route uses no chemical initiators, additives, nor catalysts, and only requires a proton beam with a kinetic energy of a few electron volts in a dry-process mode compatible with molecular-device fabrication. The reaction designs are expressed unconventionally: reaction energy is tuned by the kinetic energy of the proton beam and reactant supply is controlled precisely by the proton fluence. However, conventional considerations such as bond-strength effects on kinematic outcomes and branching-ratio statistics are also important and they can extend the reaction applicability of the kinematics concept. For example, taking advantage of the fact that COO-H bonds are stronger than C-H bonds, we show, with practical reaction conditions, synthesis results, and surface analysis using X-ray photoelectron spectroscopy and atomic force microscopy, that we can break C-H bonds without breaking COO-H and other bonds, in the production of cross-linked molecular layers with any desirable COOH concentration and with no ester nor other chemical contaminations. The new reaction-design strategies are also applicable to the synthesis of molecular layers with other functionalities such as OH, and to the synthesis of a mixture of functionalities, such as OH/COOH, with a controllable concentration ratio.     

Site‐Selective Tertiary Alkyl–Fluorine Bond Formation from α‐Bromoamides Using a Copper/CsF Catalyst System

A copper‐catalyzed site‐selective fluorination of α‐bromoamides possessing multiple reaction sites, such as primary and secondary alkyl?Br bonds, using inexpensive CsF is reported. Tertiary alkyl?F bonds, which are very difficult to synthesize, can be formed by this fluorination reaction with the aid of an amide group. Control experiments revealed that in situ generated CuF₂ is a key fluorinating reagent that reacts with the tertiary alkyl radicals generated by the reaction between an α‐bromocarbonyl compound and a copper(I) salt.     

Asymmetric hydroamination of non-activated carbon-carbon multiple bonds

The catalysed enantioselective formation of carbon-nitrogen bonds by the hydroamination reaction is reviewed. All examples deal with substrates containing non-activated carbon-carbon multiple bonds which are transformed either via intramolecular or intermolecular reactions. Structurally different complexes already provided nitrogen containing compounds/heterocycles with high enantioselectivities.     

Highly diastereoselective formation of 3,7-dioxabicyclo[3.3.0]octan-2-ones in reaction of 2-arylcyclopropanedicarboxylates with aromatic aldehydes using 1,2-zwitterionic reactivity type

A new reaction of 2-arylcyclopropane-1,1-dicarboxylates (ACDC) with aromatic aldehydes that occurs via generation of a 1,2-zwitterionic intermediate in the presence of GaCl₃ has been developed. This process results in the 3,7-dioxabicyclo[3.3.0]octane frame by addition of two aldehyde molecules to an ACDC molecule. The reaction is a complex anionic-cationic cascade process involving the formation of two C–C bonds, two C–O bonds, and five stereocenters. The process occurs with high diastereoselectivity to give only one diastereomer. A probable reaction mechanism is suggested and confirmed by NMR monitoring data.     

锰催化芳香碳-氧键的还原断裂

这里发现锰化合物能够催化芳香碳-氧键的还原断裂.就我们所知,目前还没有锰催化芳香碳-氧键断裂方面的报道.以二苯并呋喃为底物,对各种反应条件进行优化,得到的较佳反应条件为5％ Mn(OAc)2,3当量Li-A1H4,140 ℃温度,四氢呋喃溶剂.在这个反应条件下,多种芳香碳-氧键能够发生还原断裂.甲醇钠的添加能够有效地促进二苯醚等底物的反应.对反应机理进行了初步研究,结果表明反应可能经历自由基过程.     

Condensation of phenols and alcohols with 1,2-dichloroethyl-gem-dichlorocyclopropanes

Dichlorocarbenation of 3,4-dichlorobut-1-ene by Makosza method results in the stereoisomeric 1,2-dichloroethyl-gem-dichlorocyclopropanes in a 1:1 ratio. The reaction of the mixture of stereoisomeric gem-dichlorocyclopropanes with phenols and alcohols in dimethylsulfoxide in the presence of solid sodium hydroxide gives rise to the corresponding ketals containing the exocyclic double and triple bonds. In this case, the acetylene compounds dominate in the reaction products. The ratio of the reaction products containing the double and triple bonds depends on the nucleophile nature.     

Effects of tertiary amine accelerators on curing of epoxide resins

The effects of seven tertiary amine accelerators on curing of bisphenol-type epoxide resins using azelaic acid as a curing agent have been investigated. The structure of the cured resins is characterized and reaction and structure schemes are proposed. The reaction mechanism and the resulting structure of the resin depend on the basicity of the accelerator. With increasing accelerator basicity crosslinking in the cured resin increases. Characterization results indicate that the network structure consists of ether bonds or a mixture of ether and ester bonds; the linear structure consists of only ester bonds. The structure and, therefore, the properties of the cured epoxide resin may thus be regulated by selection of the amine basicity.     

Zinc‐Catalyzed [4+3] Cycloaddition with Concomitant Furan Annulation: Formation of Cyclohepta[b]Furans

A convenient zinc‐promoted [4+3] cycloaddition of a carbonyl ene–yne with simple dienes was first achieved. This reaction provided an efficient strategy to prepare various cyclohepta[b]furan rings by cascade cycloadditions. Additionally, a multicomponent reaction of dione, alkynal, and diene was also reported, which exhibited a novel strategy for selective creations of C?O bonds and C?C bonds.     

Comparison of the kinetics and thermodynamics for methyl radical addition to C=C, C=O, and C=S double bonds

The barriers, enthalpies, and rate constants for the addition of methyl radical to the double bonds of a selection of alkene, carbonyl, and thiocarbonyl species (CH(2)=Z, CH(3)CH=Z, and (CH(3))(2)C=Z, where Z = CH(2), O, or S) and for the reverse beta-scission reactions have been investigated using high-level ab inito calculations. The results are rationalized with the aid of the curve-crossing model. The addition reactions proceed via early transition structures in all cases. The barriers for addition of methyl radical to C=C bonds are largely determined by the reaction exothermicities. Addition to the unsubstituted carbon center of C=C double bonds is favored over addition to the substituted carbon center, both kinetically (lower barriers) and thermodynamically (greater exothermicities). The barriers for addition to C=O bonds are influenced by both the reaction exothermicity and the singlet-triplet gap of the substrate. Addition to the carbon center is favored over addition to the oxygen, also both thermodynamically and kinetically. For the thiocarbonyl systems, addition to the carbon center is thermodynamically favored over addition to sulfur. However, in this case, the reaction is contrathermodynamic, addition to the sulfur center having a lower barrier due to spin density considerations. Entropic differences among corresponding addition and beta-scission reactions are relatively minor, and the differences in reaction rates are thus dominated by differences in the respective reaction barriers.     

Samarium-mediated mild and facile method for the synthesis of amides

Samarium-mediated facile method for the formation of amide bonds by the reaction of acyl chlorides and amines is described. The reaction afforded high yields of the desired amides under mild and neutral conditions.